1. Field of the Invention
The present invention relates to a disk driving apparatus used mainly in the information processing field and, more particularly, to a rotor support construction for a rotor which is used in a spindle motor of the disk driving apparatus.
2. Description of the Prior Art
In recent years, a disk driving apparatus (hereinafter referred to as an apparatus) has become small in size and high in density. A typical example of a product in the field involved with the present invention is shown in FIGS. 14A and 14B. This product is a 2.5" magnetic disk driving apparatus which U.S.'s PrairieTek Co., Ltd. developed first in the world. FIGS. 14A and 14B are cited from a catalogue of the disk driving apparatus. There is a great demand for this type of apparatus, in which emphasis is placed on portability, small size, resistance to impact, low noise, and low power consumption. Naturally, there are the same demands on the spindle motor (hereinafter referred to as a motor) inside the apparatus, for rotating the disk.
One of the key components by which the above performance is determined is the bearings of the motor. Generally, ball bearings have been used as bearings, including the apparatus shown in FIGS. 14A and 14B. Hydrodynamic bearings have attracted attention as ones capable of fulfilling higher levels of the above-mentioned demands, and are being used.
The hydrodynamic bearing comprises a cylindrical shaft and a hollow cylindrical sleeve metal filled in such a manner as to have a clearance with the shaft. A herringbone groove is provided in either one of them. The hydrodynamic bearing is constructed in such a way that the clearance is filled with a fluid (oil in most cases), and the rotor is supported by pressure produced in the fluid as the rotor rotates. It has excellent features in principle as the bearing of this apparatus. For example, the volume occupied by the mechanism is small, and the noise of the rotation thereof is small because the rotor is supported via a fluid. It has excellent resistance to impact, and shaft deflection is small due to an integration effect because the load is burdened around the entire circumference of the shaft. The features explained here are, however, related to a radial bearing, and it has no thrust load support capability along the thrust. Therefore, a bearing specialized for thrust is provided separately.
In the inside of the disk driving apparatus, a magnetic head precisely follows tracks and records and reproduces signals while the head floats above a disk with a very small clearance rotating at high speeds. Therefore, the disk must not move axially during recording and reproduction. The orientation of the apparatus is not always fixed, and it may change its position when in use. In this case also, the disk must not move axially. The apparatus is frequently moved, in particular, for portable applications. If an excessive impact is applied thereto and the rotor is moved axially beyond a limit, the apparatus may be damaged. Therefore, the amount of movement of the rotor must be limited to prevent such a risk. The apparatus cannot be used unless it is so constructed as to satisfy these demands even if the hydrodynamic bearing is superior.
A two-surface opposing type construction has hitherto been proposed for a thrust bearing having performance suitable for such a case as described above. An example thereof is shown in FIGS. 13A and 13B. The figures are cited from the specification of U.S. Pat. No. 4,332,428. Although a detailed explanation is omitted, a support force along the thrust is generated at two surfaces distinguished from each other, and the directions of the forces are made to oppose to each other along the thrust, so that the position of the rotor is maintained. The periphery of gaps .delta. 1 and .delta. 2 in FIG. 13B is the thrust bearing surface.
However, this construction has problems. Since there are two thrust opposing surfaces requiring high accuracy, the apparatus is likely to become complex and therefore costly. In addition, the height of the thrust bearing mechanism increases. An increase in the height of the bearing causes the height which can be allocated for the radial hydrodynamic bearing to decrease, and thus it may be impossible to form the hydrodynamic fluid bearing. Further, loss torque usually becomes larger than that in a single surface opposing type, causing the consumption of electric current of the apparatus to increase. The various demands, described above, cannot always be satisfied except for support performance.
Among the single surface opposing types, in which the support surface is formed of a set of thrust bearings, are the following: one in which the spherical surface and the flat surface contact each other at the rotation center as regards the condition of the support surface; and one in which the flat surfaces rotate while they oppose each other, and a hydrodynamic pressure is generated by a means such as a spiral groove provided in one of them in order to support the rotor. However, since the single surface opposing type has a capability to support loads only in one direction, it has the following two problems: (1) the position of the rotor is not stable and likely to float; and (2) the rotor is displaced when an excessive acceleration is applied thereto.